In recent years, research and development of a light emitting element using a light emitting organic compound, have been actively carried out. In a basic structure of such a light emitting element, a layer containing a light emitting organic compound is sandwiched between a pair of electrodes. By applying voltage to the light emitting element, electrons and holes are respectively injected in the layer containing the light emitting organic compound from the pair of electrodes, and thus current flows through the light emitting element. Then, by recombining these carriers (electrons and holes), the light emitting organic compound is excited, and light is emitted when returning to a ground state from the excited state. In accordance with the above mentioned mechanism, the light emitting element is referred to as a current excitation type light emitting element. Further, as kinds of excited states produced by an organic compound, there are a singlet excited state and a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence and light emission from the triplet excited state is referred to as phosphorescence.
Such a light emitting element can be thinly manufactured with lightweight using an organic thin film with a thickness of about 0.1 μm, and this is a great advantage. Since time from injecting carries to emitting light is about several micro seconds or less, one feature of the light emitting element is that the light emitting element has extremely high response speed. It is thought that these characteristics are preferable for a flat panel display element. In addition, such a light emitting element is formed in a film form, by forming a light emitting element with a large area, light emission in a sheet form can be easily obtained. These characteristics are difficult to be obtained in a point light source typified by an incandescent filament lamp or an LED, or in a linear light source typified by a fluorescent lamp, and therefore, the light emitting element can be highly utilized as a light source in a sheet form, which can be applied to lighting and the like.
As mentioned above, there are high expectations for application of a current excitation type light emitting element using a light emitting organic compound, in a light emitting device, lighting, and the like. However, the current excitation type light emitting element still has many problems. As one of the problems, reduction in power consumption can be given. In order to reduce the power consumption, it is important to reduce driving voltage of a light emitting element. Since light emission intensity of such a current excitation type light emitting element is determined based on the amount of current flowing through the light emitting element, in order to reduce driving voltage, it is necessary to feed a large amount of current at low voltage.
As a method for reducing driving voltage, an attempt of providing a buffer layer between an electrode and a layer containing a light emitting organic compound has been carried out in the past. For example, it is known that by providing a buffer layer formed using polyaniline (PANI) doped with camphorsulfonic acid, between indium tin oxide (ITO) and a light emitting layer, driving voltage can be reduced (e.g., see Non Patent Document 1). It is explained that this is because the PANI has an excellent carrier injecting property with respect to the light emitting layer. Further, in Non Patent Document 1, it is assumed that the PANI, which is the buffer layer, is a part of an electrode.
However, as disclosed in Non Patent Document 1, the PANI has a problem that transmittance is decreased when a thickness of the PANI is increased. Specifically, it is reported that when a thickness of the PANI is set to be about 250 nm, the transmittance is less than 70%. That is, since a material used for a buffer layer itself has a problem in transmittance, light generated inside of an element cannot be extracted efficiently.
According to Patent Document 1, an attempt of increasing luminance for current density, i.e., current efficiency has been carried out by connecting light emitting elements (which are described as light emitting units in Patent Document 1) in series. In the Patent Document 1, a layer in which an organic compound and metal oxide (concretely, vanadium oxide and rhenium oxide) are mixed, is applied in a connection portion of the light emitting elements, which connected in series. It is considered that this layer can inject holes and electrons to a light emitting unit.
However, as seen from an embodiment mode of the present invention, in the mixed layer of the organic compound and the metal oxide, which is disclosed in the Patent Document 1, a large absorption peak is generated not only in an infrared region but also in the vicinity of 500 nm, and therefore, there is a problem in a light transmitting property. Accordingly, light generated inside of an element cannot be extracted efficiently, thereby reducing light emitting efficiency of the element.    [Non Patent Document 1]: Y. Yang et al., Applied Physics Letters, vol. 64 (10), 1245-1247 (1994)    [Patent Document 1]: Japanese Patent Application Laid-Open No. 2003-272860